The invention relates to the field of micro-turbines and, in particular, to starting systems for microturbines.
Microturbines are small turbines typically used for on-site power generation. They are generally applied as back-up or auxiliary power sources for office buildings, retail store, small manufacturing plants, homes and many other facilities. These facilities have traditionally been powered by electric utilities via a grid of power distribution lines. Microturbines enable these facilities to generate electrical power at their own sites and thereby avoid being solely dependent on conventional electrical power grids and utilities. Microturbines may also generate power at less cost and/or more reliably than the electrical power provided over the grid by electrical power utilities.
Microturbines require a starter. This starter usually includes a powerful electric machine and a power source for the electric machine, such as a battery. The electric machine may be configured (1) to operate as a motor to start the turbine, and (2) as a generator for a load that is driven by the microturbine once the microturbine has been started. During a start-up phase, the electric machine (operating in motor mode) rotates the microturbine until it has been accelerated to a rotational speed rapid enough to enable the microturbine to operate on its own and drive the electric machine (then operating in generator mode).
A conventional controller for starting a microturbine is disclosed in U.S. Pat. No. 6,020,713 (""713 Patent) that describes a pulse-width modulator (PWM) inverter that operates as an adjustable speed motor drive to start the microturbine. An electrical machine coupled to the PWM inverter is configured as a motor to drive the microturbine during startup and is then configured as a generator that is driven by the microturbine. The PWM inverter described in this ""713 Patent varies both the frequency and amplitude of the AC power that it generates to start the microturbine. During a start-up phase of the microturbine, the PWM inverter is powered by a DC bus that is charged to a constant DC voltage. Thus, the PWM inverter is required to convert DC power at constant voltage to variable voltage and variable frequency AC power.
It may not be desirable to rely on a PWM inverter to generate both the variable AC voltage and variable AC frequency needed to start a microturbine. Similarly, it may not be desirable for a single power circuit component, e.g., a PWM inverter, to vary both the frequency and amplitude of the AC power generated to start a microturbine. There is a need for a power circuit for starting a microturbine that does not rely on a PWM inverter to vary both the frequency and voltage of AC power applied to start the microturbine.
Moreover, the voltage applied to the electric machine, when operating in motor mode, must be correlated with the frequency of the applied voltage to maintain the desired flux. This correlation is traditionally accomplished in two steps using two distinct power circuits, i.e. the battery charger and the DC-AC converter (inverter).
There is a need for a power circuit coupled to a microturbine that does not require distinct circuit components for starting and operating a microturbine.
The present invention relates to power circuits that start a microturbine and later couple a microturbine to an electrical load. A power circuit generates variable DC voltage which is subsequently converted to AC by another power circuit. The frequency of the AC power generated by the inverter (DC-AC converter) is varied in a controlled manner bearing a fixed relationship with the DC voltage, such that it leads to the acceleration of the microturbine. Accordingly, the AC power applied to start the microturbine has both variable frequency and variable voltage.
In an exemplary embodiment of the invention, a battery provides a source of DC power for starting a microturbine. The constant voltage of the battery is converted by a buck-boost chopper circuit to variable voltage DC power. The buck-boost chopper steps-up the voltage of the DC power through a series of voltage levels that are sequentially applied to drive the microturbine during a start-up phase via the DC-AC converter. The buck-boost chopper applies the variable DC voltage to a capacitive DC bus that distributes the variable DC voltage to other power circuit components. The variable DC power on the DC bus is converted to AC power using a DC-to-AC converter. This converter generates AC power having a variable frequency. The frequency of the AC power is sequentially increased to match (or slightly lead) the desired accelerating starting speed of the microturbine.
The combination of a buck-boost chopper circuit, capacitive DC bus, and a DC-to-AC converter provides a circuit arrangement to start a microturbine. This power circuit arrangement satisfies the need for a power circuit that does not exclusively rely on a PWM scheme to both vary the frequency and voltage of generated AC power applied to start a microturbine.
The invention offers several advantages over prior start-up power circuits including that a single power control circuit component is nor relied on to vary both the frequency and voltage of the AC power applied to start a microturbine. When the voltage waveforms generated at the output of the inverter are of the six-step type, the switching losses are lower. In addition, the control scheme of the present power control circuit can be simplified as compared to the PWM inverter control schemes.